bass strngs

[pianotune05 at comcast.net]

I have a question about strings as well.  My question is this.  What makes a string have a rubber band sound no matter how much it's tuned.  The D below F3 has a "rubber band" sound.  Is this because the last tuner used a universal bass string?  What is the difference between a universal bass string and one duplicated from the old one?  As long as the string is the same size thickness is that the key?  If the old string is missing, where would I find one for this piano?  It's my church's piano, and I figured that when I come back on a break from Emil Fries, I can replace the string.  Thanks guys 
Marshall Soon to be a heavy hitter like you 
Villa Park